David Rolph, University of Sydney, Faculty of Law, has published Defamation by Social Media at 117 Precedent 16 (2013). Here is the abstract.

Social media have transformed communications, allowing users to generate and disseminate content widely. This carries with it a heightened risk of defamation. Cases involving defamation by social media are starting to be decided by Australian and English courts. This article reviews recent decisions and examines the issues presented by defamation and social media, particularly the anonymity of users and the legal means to overcome it and the liability of internet intermediaries, such as internet service providers and search engines.